Automatic repeat-request (ARQ) is an error-control technique used in many wireless networks. With ARQ, a receiver of data transmissions sends acknowledgements (ACKs) or negative acknowledgments (NACKs), referred to “ARQ feedback,” to inform the transmitter of whether each message has been correctly received. Incorrectly received messages, as well as messages that aren't acknowledged at all, can then be re-transmitted.
Hybrid ARQ (HARQ), which can be understood as a refined form of ARQ, combines the basic ARQ techniques described above with forward error-correction (FEC) coding of the data messages, to improve the ability of the receiver to receive and correctly decode the transmitted messages. As with conventional ARQ, receivers employing HARQ send ACKs and NACKs, as appropriate, after each attempt to decode a message. These ACKs and NACKs are referred to as “HARQ feedback.”
The fourth-generation system widely known as LTE (for Long-Term Evolution) and more formally known as the Evolved UMTS Terrestrial Radio-Access Network (E-UTRAN) has been developed and deployed by members of the 3rd-Generation Partnership Project (3GPP). For downlink transmissions in LTE today, HARQ feedback is sent from the UE (user equipment—3GPP terminology for a mobile terminal) to the network on either the Physical Uplink Control Channel (PUCCH) or the Physical Uplink Shared Channel (PUSCH), depending on whether or not the UE has been scheduled for uplink PUSCH transmission.
For each downlink transmission to the UE, the UE will transmit an ACK or NACK, depending on whether or not the transmission was correctly received. The network can, apart from detecting ACK or NACK, also draw the conclusion that the UE did not properly receive the corresponding downlink assignment, and thus did not even attempt to demodulate and decode the transmitted data message, if the network detects no feedback from the UE when anticipated. Note, however, that a missed ACK/NACK detection on the network side could also be due to the network's failure to receive it properly, even though it was transmitted by the UE.
A NACK received by the network will trigger a retransmission, whereas the reception of an ACK allows the corresponding HARQ transmit (Tx) process to be flushed and re-used. That a HARQ Tx process has been flushed and is being re-used is indicated to the UE from the network side by toggling the NDI (New Data Indicator) flag in the next downlink assignment for the corresponding HARQ process, along with the new data. This, in turn, will cause the UE to flush the HARQ receive (Rx) process and associate the newly received data with the current HARQ Rx process.
As described above, the failure of the network to receive an ACK/NACK from a UE when expected is an indication of a probable failure by the UE to correctly decode the downlink assignment message, which in LTE is sent on the Physical Downlink Control Channel (PDCCH) or enhanced PDCCH (ePDCCH). Detecting this condition at the network may thus be used for the purpose of PDCCH/ePDCCH link adaptation—i.e., for adapting the robustness of the physical control channel that carries the downlink grant.
While LTE is based on a fixed (or semi-static) division between uplink and downlink, other systems, as well as future versions of LTE, may be based on a dynamically scheduled division between uplink and downlink. In such systems, there may not be a one-to-one correspondence between downlink transmissions and HARQ feedback messages.
In upcoming enhancements of LTE, the assumed mode of operation is dynamic time-division duplexing (TDD), where subframes can be dynamically allocated for uplink or downlink use. With this approach, the number of HARQ processes may increase quite significantly, because, for example, a relatively large number of transmission-time-intervals (TTIs) in a row may be assigned to the downlink. This causes the number of needed HARQ processes to grow, since there is a long gap between feedback opportunities in the uplink, to free up HARQ processes.